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Originally Posted by Battara
If the top part was polished bright and then you give it a light etch, it would indeed not darken as much as the bottom part.
The idea that there is differential heat treatment is possible, but I think less likely.
Great piece in any case!  |
Jose, it seems to me from experience that if you polish the upper side bright and etched the whole thing, the lower side will darken if the composition (crystalline structure) of the steel gives it that tendency, regardless of what you do to the surface. Wootz reacts to etchants according to its own nature.
Differential heat treating was not only possible, but it was indeed widely practiced by smiths working in "pulad jawhar" (wootz) in Iran and India. I have polished many blades which show this effect. The delineation in many cases is quite crisp, and with a skilful polish and etch, on an exceptional blade you can see a narrow, cloudy crystalline border between the zones. This is akin to an identical effect called the habuchi which is quite distinct on some Japanese blades and is a much-desired aesthetic effect. The fact that its appearance is not limited by culture or geography is also borne out by its emergence when I have polished some outstanding Burmese dha and knife blades as well. Some of those are truly dramatic.
Differential heat treating of edges may be done with or without the use of a layer of refractory clay to cover the body of the blade and provide a sort of heat-sink (this controls the amount of heat absorbed in each area, and therefore the hardness after the blade is quenched) . Moro barong blades, for example, were treated without the layer; the result is not artistic but is the functional result leaves nothing to be desired. In such cases, the blade is merely held edge-down and moved rapidly to and fro in the forge to bring the edge only up to heat, followed by dunking in water or oil.
Most people think that the use of a refractory clay layer is a specifically Japanese technique. Wrong! Read this from the treatise, "On Iron", by Mohammed ibn Ahmed al-Biruni (10th cent. AD):
"During quenching, they [the smiths of Iran and India] coat the broadside [full width] of the sword with suitable clay, cow dung, and salt in the form of a paste, and test [i.e. mark out] the place of quenching at two fingers from the two sides of the cutting edges. Then they heat it by blowing [the hearth with bellows], the paste boils, and they quench it and cleanse its surface from the coating..." (trans. Brian Gilmour and Robt Hoyland in MEDIEVAL ISLAMIC SWORDS AND SWORDMAKING, Oxford 2006, p 152.
In my more limited experience in working with the steel blades of late medieval and renaissance-era European swords I detect signs of a similar albeit less-developed concept at play. I'd love to investigate this further on damaged blades as they become available, since the aesthetic standards of collectors in these fields tends to frown on polishing and etching blades.